The timing of marriage and family formation affects everybody’s family life cycle. Macro-level socio-economic factors, personal characteristics and the availability of appropriate spouses may contribute to early or late marriage. The increase of age at marriage in contemporary era, on the other hand, has caused many social, economic and demographic changes. Using data from a survey of social and economic characteristics of households, carried out by Iran Statistical Centre in 2001, this study examines the factors affecting the rise of men’s and women’s ages at marriage in Iran. Data showed recent marriages take place at older ages. Educational attainment, especially at higher levels, was associated with higher ages at marriage. Urbanization, women’s participation in labor force, free choice in spouse selection, gender equity and not being a relative of the spouse showed positive relations with the age at marriage. Ethnic affiliations had negative and positive effects on the timing of marriage. Multivariate analysis showed the significant effects of duration of marriage, education, place of residence, ethnicity and kinship of spouses. The improvement of women status, emerging new occupational systems, changes in the proportions of population looking for job and marriage, the availability of appropriate spouses, and changes in norms and values related to marriage and family formation may explain these effects.